Mesogenic Schiff's Base Esters with a Methoxyethyl Tail

Mesogenic Schiff’s Base Esters with a Methoxyethyl Tail

A. K. PrajapatiN. L. BondeApplied Chemistry Department, Faculty of Technology and Engineering,Kalabhavan M. S. University of Baroda, Vadodara, India

Two new mesogenic homologous series, each consisting of a methoxyethyl tail and aSchiff ’s base central linkage, have been synthesized by condensing 2-methoxyethyl4-aminobenzoate with different 4-n-alkoxybenzaldehydes or 4-n-alkoxybenzoyloxy-benzaldehydes to give series I and series II, respectively, and their mesomorphicbehavior was studied. In series I, 2-methoxy ethyl 4-(40-n-alkoxybenzylidine)amino-benzoates, the n-butoxy derivative is nonmesogenic whereas higher homologuesexhibit the SmA mesophase. In series II, 2-methoxyethyl 400-[4-(40-n-alkoxy-benzoyloxy)benzylidene]aminobenzoates, all the twelve compounds synthesizedexhibit mesomorphism. Methoxy to n-octyloxy derivatives exhibit an enantiotropicnematic mesophase. The smectic A mesophase commences from n-propoxy deriva-tive and persists up to the last homologue synthesized. The mesomorphic propertiesof both series are compared with each other and also with the properties ofother structurally related series to evaluate the effects of the ethoxyethyl chain onmesomorphism.

Keywords: liquid crystals; methoxyethyl tail; Schiff ’s base esters

1. INTRODUCTION

Liquid-crystalline properties are highly sensitive to the molecularshape, lateral substituent, terminal substituent, hydrogen bonding,bridging group, breadth-to-length ratio etc.; all these effects are verywell studied and reported in the literature [1–3].

The influence of the terminal substituent has a particular impor-tance. Early reviews suggest that molecules of mesogenic compoundsshould contain moderately dipolar terminal groups. It has alsobeen found that terminally substituted compounds exhibit higher

Address correspondence to A. K. Prajapati, Applied Chemistry Department, Facultyof Technology and Engineering, Kalabhavan M. S. University of Baroda, P.B. No. 50,Vadodara 390 001, India. E-mail: [email protected]

Mol. Cryst. Liq. Cryst., Vol. 461, pp. 15–28, 2007

Copyright # Taylor & Francis Group, LLC

ISSN: 1542-1406 print=1563-5287 online

DOI: 10.1080/15421400600914882

15

mesophase stability than unsubstituted compounds. It is also verywell known that the terminal substituent increases the overallpolarizability of the molecule without increasing its breadth and henceit has the higher mesophase stability. Generally in a liquid-crystallinecompound, the end groups are compact units like –CN, �NO2, orflexible chains such as n-alkyl or n-alkoxy [4–8]. There have beenfew research papers indicating a branch chain at the terminalposition [9–11]. Booth et al. have described the synthesis and charac-terization of three achiral-branched alkyl 4-(40-dodecyloxybiphenyl-4-carbonyloxy)-3-fluorobenzoates that show SmCalt and overlying SmAphases [12].

There are also very few examples that have a broken alkoxy ter-minal chain. Weygand et al. [13] have reported mesogenic propertiesof an alkyl chain combining two ether functions as a terminal sub-stituent. Chiang et al. [14,15] studied the effect of ethoxyethoxyethoxyand butoxyethoxyethoxy tails on mesomorphism. They reported thatthe latter unit has greater Smetic C (SmC�) thermal stability thanthe former unit. Earlier we have reported methoxyethyl and ethox-yethyl esters of 4(40-n-alkoxybenzoloxy)benzoic acids [16] as well asmethoxyethyl trans-4(40-n-alkoxybenzoyloxy)-a-methylcinnamates[17]. All the three mesogenic homologous series exhibited the smecticA mesophase at ambient temperatures. To study the effect of an azocentral linkage on thermal stability, we have reported mesogenichomologous series of azomesogens with an ethoxyethyl [18] and amethoxtethyl tail [19]. We [20] have also synthesized a mesogenichomologous series of Schiff ’s base esters containing an ethoxyethyltail and evaluated the effect of a Schiff ’s base linkage and an ethoxy-ethyl tail on mesomorphism. From all these studies we have observedthat such terminal chains adversely affect the mesophase thermalstability but do not eliminate mesomorphism. In continuation of ourwork on a substituted ethyl tail, we have synthesized two new meso-genic homologous series with a methoxyethyl tail and evaluated theeffect of a methoxyethyl tail and a Schiff ’s base linkage onmesomorphism.

2. EXPERIMENTAL

2.1. Materials

4-Hydroxybenzoic acid, 4-hydroxybenzaldehyde, 4-aminobenzoic acid,1-bromoalkanes, potassium hydroxide, potassium carbonate, thionylchloride, and 2-methoxyethanol were used as received. Solvents weredried and distilled prior to use.

16 A. K. Prajapati and N. L. Bonde

https://www.researchgate.net/publication/260821203_Introduction_to_Liquid_Crystals_Chemistry_and_Physics?el=1_x_8&enrichId=rgreq-cb03783ebb5e9ec6a5cc1e04b5da94b8-XXX&enrichSource=Y292ZXJQYWdlOzIzMzMxNzYzNDtBUzoxMjkyNzg0MDIzNzE1ODRAMTQwNzgzMzc4MTIxNQ==

2.2. Characterization

Microanalyses of the compounds were performed on a Perkin ElmerSeries 2400 elemental analyzer. IR spectra were determined usingKBr pellets, using a Shimandzu IR-408 specrophotometer. 1H NMRspectra were obtained with a Bruker Dpx 200 spectrometer, usingtetramethylsilane (TMS) as an internal reference standard. Thechemical shifts are quoted as d (parts per million) downfield fromthe reference. CDCl3 was used as a solvent. The liquid-crystallineproperties of the compounds were investigated on a Leitz Laborlux12 POL microscope equipped with a heating stage. The enthalpies oftransitions reported as J=g were determined from thermogramsobtained on a Universal V3.0G TA instrument adopting a rate of5�C=min. The calorimeter was calibrated using pure indium as thestandard.

2.3. Synthesis

The synthetic routes of both series are illustrated in Scheme 1.2-Methoxyethyl 4-aminobenzoate (1) was synthesized by the esteri-

fication of 4-aminobenzoic acids with 2-methoxyethanol [21]. The esterwas crystallized repeatedly until a constant melting point wasobtained, resulting in a white solid product. Mp: 58�C; yield: 67%.Elemental analysis: found C 61.74, H 6.82, N 7.05; C10H13NO3

requires C 61.53, H 6.67, N 7.17%. The Fourier transform infrared(FTIR) spectrum of the compound showed two bands for N�H stretch-ing vibrations of a free amino group at 3453 and 3360 cm�1. The ester(�COO�) stretching vibrations were seen at 1686 cm�1. The other sig-nals observed were at 1634 (N�H bending vibration), 1607, 1520,1445, 1310, 1275, 1310, 1105, 840 cm�1.

4-n-Alkoxybenzaldehydes (2) were prepared by the method of Grayand Jones [22].

2-Methoxyethyl 4-(40-n-alkoxybenzylidine)aminobenzoates (Series I)was prepared as follows: nine Schiff ’s bases of series XI were synthe-sized by condensing equimolar quantities of 2-methoxyethyl 4-amino-benzoate (1) and the appropriate 4-n-alkoxybenzaldehyde (2) inboiling ethanol. All the Schiff ’s bases were crystallized from methanoluntil constant transition temperatures were obtained. Yield: 83–91%.The elemental analysis of all the compounds were found to be satisfac-tory, and all are listed in Table 1. IR and 1H NMR spectral data ofn-dodecyloxy and n-tetradecyloxy derivatives as representativemembers are given below.

Mesogenic Schiff’s Base Esters 17

SCHEME 1 (a) (i) Dry HCL, CH3OC2H4OH, reflux; (ii) R Br, K2CO3, acetone,reflux; (iii) EtOH, 1, AcOH (cat.), reflux; Series I: R ¼�CnH2nþ 1, n ¼ 4–8, 10,12, 14, and 16; (b) (i) RBr, KOH, EtOH; (ii) SOCl2 (excess); (iii) 4-hydroxy ben-zaldehyde, pyridine; (iv) cold aqueous 1:1 HCl; (v) EtOH, 1, AcOH (cat.),reflux; Series II: R ¼�CnH2nþ 1, n ¼ 1–8, 10, 12, 14, and 16.


2-Methoxyethyl 4-(40-n-dodecyloxybenzylidine)-aminobenzoate

IR spectrum (KBr) nmax=cm�1: 2921 (nC�H, aliphatic), 2847,1702(�COO�), 1594(�CH=N�), 1511, 1416, 1168, 1099, 839.

1H NMR spectrum (CDCl3, 200 MHz): d 8.35 (s, 1H,�CH=N�), 8.08 (d,J ¼ 8.7 Hz, 2H, ArH at C-2 and C-6), 7.84 (d, J ¼ 8.4 Hz, 2H, ArH at C-20

and C-60), 7.19 (d, J ¼ 8.7 Hz, 2H, ArH at C-3 and C-5), 6.98 (d, J ¼ 9 Hz,2H, ArH at C-30 and C-50), 4.48 (t, J ¼ 4.4 Hz, 2H, �COOCH2C�), 4.03(t, J ¼ 4.4 Hz, 2H, Ar�O�CH2�), 3.74 (t, J ¼ 5.1 Hz, 2H,�COOCCH2�),3.44 (s, 3H, �O�CH3), 1.82 (m, 2H, ArOCCH2�), 1.27–1.58 (m, 18H,9x�CH2�), 0.88 (t, J ¼ 6 Hz, 3H, �CH3).

2-Methoxyethyl 4-(40-n-tetradecyloxybenzylidine)-aminobenzoate

IR spectrum (KBr) nmax=cm�1: 2918 (nC�H, aliphatic), 2850, 1718(�COO�), 1593 (�CH=N�), 1512, 1419, 1168, 1109, 841.

TABLE 1 Elemental Analysis for Series I and II Compounds

Compoundno.

R ¼ CnH2nþ1

nMolecularformula

% Required (found)

C H N

Series I1 4 C21H25NO4 74.34(74.61) 7.37(7.69) 4.13(3.77)2 5 C22H27NO4 74.79(74.92) 7.65(7.50) 3.97(3.54)3 6 C23H29NO4 75.20(74.82) 7.9(7.53) 3.81(3.62)4 7 C24H31NO4 75.59(75.83) 8.14(8.32) 3.67(3.83)5 8 C25H33NO4 75.95(75.61) 8.35(8.61) 3.54(3.54)6 10 C27H37NO4 76.6(76.87) 8.75(8.98) 3.31(3.04)7 12 C29H41NO4 77.16(77.61) 9.09(8.84) 3.1(3.29)8 14 C31H45NO4 77.66(77.53) 9.39(9.16) 2.92(3.18)9 16 C33H49NO4 78.11(77.69) 9.66(9.73) 2.76(2.43)Series II10 1 C25H23NO6 69.28(69.59) 5.31(5.62) 3.23(3.46)11 2 C26H25NO6 69.8(69.54) 5.59(5.30) 3.13(2.84)12 3 C27H27NO6 70.28(70.49) 5.86(5.42) 3.04(2.93)13 4 C28H29NO6 70.73(70.81) 6.11(5.92) 2.95(3.29)14 5 C29H31NO6 71.17(70.98) 6.34(6.65) 2.86(2.54)15 6 C30H33NO6 71.57(71.36) 6.56(6.61) 2.78(2.97)16 7 C31H35NO6 71.95(72.37) 6.78(6.99) 2.71(2.94)17 8 C32H37NO6 72.32(72.04) 6.97(7.02) 2.64(2.17)18 10 C34H41NO6 72.99(72.81) 7.33(6.91) 2.5(2.84)19 12 C36H45NO6 73.59(73.21) 7.67(7.38) 2.39(2.03)20 14 C38H49NO6 74.15(74.53) 7.97(7.81) 2.28(2.61)21 16 C40H53NO6 74.65(74.79) 8.24(8.51) 2.14(1.80)


1H NMR spectrum (CDCl3, 200 MHz): d 8.35 (s, 1H, �CH=N�), 8.08(d, J ¼ 8.7 Hz, 2H, ArH at C-2 and C-6), 7.84 (d, J ¼ 8.0 Hz, 2H, ArH atC-20 and C-60), 7.19 (d, J ¼ 8.7 Hz, 2H, ArH at C-3 and C-5), 6.98 (d,J ¼ 9 Hz, 2H, ArH at C-30 and C-50), 4.48 (t, J ¼ 4.6 Hz, 2H,�COOCH2C�), 4.03 (t, J ¼ 6.4 Hz, 2H, Ar�O�CH2), 3.74 (t, J ¼ 5.1 Hz,2H, �COOCCH2�), 3.44 (s, 3H, �O�CH3), 1.82 (m, 2H, ArOCCH2�),1.27–1.58 (m, 22H, 11x�CH2�), 0.88 (t, J ¼ 6 Hz, 3H, �CH3).

4-n-Alkoxybenzoic acid (3) and 4-n-alkoxybenzoyl chloride (4) weresynthesized by the method of Dave and Vora [23].

4-n-Alkoxybenzoylxoy-40-benzaldehydes (5) were synthesized bythe method of Dave and Kurian [24].

2-Methoxyethyl 400-[4-(40-n-alkoxybenzoyloxy)benzylidene]amino-benzoates (Series II) were synthesized as follows:

The corresponding 4-n-alkoxybenzoylxoy-40-benzaldehyde (0.01 mol)(5) was dissolved in dry ethanol and was added dropwise to the round-bottom flask containing 2-methoxyethyl 4-aminobenzoate (0.01 mol)(1), which was previously dissolved in dry ethanol. The resulting reac-tion mixture was refluxed for 2 h. The crude product was repeatedlycrystallized from dry ethanol to give a white solid compound. Theelemental analysis of all the compounds were found to be satisfactoryand are recorded in Table 1. IR and 1H NMR spectral data of n-tetra-decyloxy derivative as a representative member are given below.

2-Methoxyethyl 400-[4-(40-n-tetradecyloxybenzoyloxy)-benzylidene]aminobenzoate

IR spectrum (KBr) nmax=cm�1: 2919 (nC�H, aliphatic), 2851, 1711(�COO�), 1599 (�CH=N�), 1513, 1414, 1171, 1116, 844.

1H NMR spectrum (CDCl3, 400 MHz): d 8.44 (s, 1H, �CH=N�), 8.1(m, 4H, ArH at C-20, C-60, C-200 and C-600), 7.96 (d, J ¼ 8.7 Hz, 2H,ArH at C-2 and C-6), 7.36 (d, J ¼ 8.6 Hz, 2H, ArH at C-300 and C-500),7.26 (d, J ¼ 9 Hz, 2H, ArH at C-3 and C-5), 7.0 (d, J ¼ 9 Hz, 2H,ArH at C-30 and C-50), 4.5 (t, J ¼ 4.7 Hz, 2H, �COOCH2�), 4.02(t, J ¼ 6.4 Hz, 2H, ArOCH2�), 3.75 (t, J ¼ 5.0 Hz, 2H, �COOCCH2�),3.4 (s, 3H, �OCH3), 1.82 (m, 2H, Ar�O�C�CH2�), 1.26 (m, 22H,11x�CH2�), 0.9 (t, 3H, �CH3).

3. RESULTS AND DISCUSSION

3.1. Optical Microscopic Studies

As a preliminary investigation, the mesophases exhibited by series Iand II were examined using a polarizing optical microscope. Thin filmsof the samples were obtained by sandwiching them between a glassslide and a cover slip.


Series IFirst member synthesized (n ¼ 4) of the series I is nonmesogenic.

On cooling from the isotropic liquid in an ordinary slide, n-pentyloxyto n-hexadecyloxy derivatives of the series I show the focal conictexture characteristic of SmA phase.

Series IIOn cooling from the isotropic liquid, the methoxy to n-octyloxy deri-

vatives of series II show the threaded=marble texture characteristic ofnematic mesophase, which on further cooling (n� 3), transforms intothe focal conic texture of a SmA mesophase. On cooling from theisotropic liquid, the n-decyloxy to n-hexadecyloxy derivatives showfocal conic textures of the SmA mesophase.

3.2. DSC Studies

As representative cases, the associated enthalpies of transition of then-dodecyloxy derivative of series I as well as the n-decyloxy and then-tetradecyloxy derivatives of series II were measured by differentialscanning calorimetry (DSC). Data are recorded in Table 2. Enthalpychanges of the various transitions agree well with the existing relatedliterature value [25].

3.3. The Phase Behavior

Series I: 2-Methoxyethyl 4-(40-n-alkoxybenzylidine)-aminobenzoates

Nine compounds were synthesized, and their mesogenic propertieswere evaluated. The n-butoxy derivative is nonmesogenic, but then-pentyloxy derivative exhibits a monotropic SmA phase, whereasfor n-hexyloxy, all the members exhibit an enantiotropic SmA meso-phase. The transition temperatures are recorded in Table 3. A plotof transition temperatures against the number of carbon atoms inthe alkoxy chain is shown in Fig. 1. The SmA–I curve rises steeplyat first, which reaches to maxima for the n-heptyloxy derivative beforefalling progressively.

Series II: 2-Methoxyethyl 400-[4-(40-n-alkoxybenzoyloxy)-benzylidene]aminobenzoates

All the twelve compounds synthesized exhibit mesomorphism. Themethoxy to n-octyloxy derivatives exhibit an enantiotropic nematicmesophase. The SmA mesophase commences from the n-propyloxy


derivative and persists up to the final homologue synthesized. Thetransition temperatures are recorded in Table 3. A plot of transitiontemperatures against the number of carbon atoms in the alkoxy chain(Fig. 2) shows smooth falling tendencies for the N-I and the SmA–I

TABLE 3 DSC Data for Series I and II

Series Compound no. Transition Peak temp. (�C) DH (J=g) DS(J=gK)

I 7 Cr-Sm A 57.00 41.30 0.2464Sm A-I 67.50 2.06 0.0311

II 18 Cr-Sm A 115.56 18.65 0.0480Sm A-I 174.07 2.19 0.0050

20 Cr-Sm A 118.18 17.80 0.0460Sm A-I 156.83 3.96 0.0120

TABLE 2 Transition Temperatures (�C) of the Series I and II Compounds

Compoundno.

R ¼�CnH2nþ1

n Cr SmA N I

Series I1 4 . 71 — — — — .2 5 . (61)� . — — 75 .3 6 . 63 . — — 77 .4 7 . 59 . — — 83 .5 8 . 57 . — — 81 .6 10 . 42 . — — 77 .7 12 . 58 . — — 68 .8 14 . 61 . — — 67 .9 16 . 60 . — — 66 .

Series II10 1 . 161 — — . 219 .11 2 . 158 — — . 217 .12 3 . 136 . 156 . 215 .13 4 . 134 . 154 . 211 .14 5 . 135 . 148 . 207 .15 6 . 127 . 146 . 202 .16 7 . 120 . 145 . 197 .17 8 . 117 . 148 . 192 .18 10 . 115 . — — 174 .19 12 . 114 . — — 163 .20 14 . 117 . — — 158 .21 16 . 101 . — — 146 .

Notes. ( )�, monotropic value; Cr, crystalline solid; Sm A, Smectic A phase; Sm C,Smectic C phase; N, Nematic phase; I, isotropic liquid phase; ., phase exists; and —,phase does not exist.


transition temperatures with increasing chain length. The SmA–Ntransition temperatures a show smooth falling and then a risingtendency for the n-octyloxy homologue of the series.

FIGURE 1 Phase behavior of series I.

FIGURE 2 Phase behavior of series II.


3.4. Mesogenic Properties and Molecular Constitution

It is well known that thermotropic liquid crystals are highly sensitiveto the molecular constitution. It is of prime importance from thechemist’s point of view to find the effect of alteration in molecularcore to the mesogenic properties of the compound. As mentionedpreviously, the thermal stability and mesophase length as a measureof mesomorphism can be correlated with the molecular constitutionof the compounds.

Table 4 and Scheme 2 summarize the mesophase range (widthof mesophase), thermal stability, and molecular structures of then-hexyloxy derivative of present series I and II (compounds 3 and15) and structurally related compounds A [16], B [26], C [27], D[20], and E [26]. Table 4 indicates that compound 3 exhibits an enan-tiotropic SmA mesophase, whereas compound 15 exhibits on enantio-tropic SmA phase as well as the enantiotropic nematic mesophase. Thesmectic mesophase range and smectic phase thermal stability ofcompound 15 is greater by 9�C and 73�C than those of compound 3.Reference to the molecular structure of both the compounds indicatesthat compound 15 differs from compound 3 in the number of benzenerings and central linkages. Compound 15 is longer than that of com-pound 3 because of the additional phenyl ring and ester central link-age. Gray [28] has explained that the increase in the length of themolecule, as a result of its polarizability, increases the intermolecularcohesive forces that would be responsible for the induction of thenematic mesophase and the higher smectic phase thermal stabilitiesof compound 15 than of compound 3.

The smectic mesophase range and smectic mesophase thermalstability of compound 3 is higher by 6�C and 34�C than those of

TABLE 4 Comparison of Mesophase Length (�C), Thermal Stabilities (�C),and Molecular Structure of Compounds 3, 16, A, B, C, D, and E

Compoundno.

Mesophase length Thermal stabilitiesCommencement of

Smectic phaseSm N Sm N

3 14 — 77 — 5A 8 — 43 — 5B 23 — 88 — 5C 16.5 — 84.5 — 215 23 52 150 202 3D 78 6.5 183.5 190 2E 21 61 190 251 3


compound A (Table 4); both the compounds differ only at the centrallinkage. Compound 3 has an azomethine (�CH=N�) central linkage,whereas compound A has an ester (�COO�) central linkage. The azo-methine central linkage is more coplanar and provides such packing tothe molecules that the smectic phase thermal stability increases. It isalso known that the liquid-crystalline properties are enhanced mostwhen all the rings are conjugated; i.e., the liquid crystal transitiontemperatures are highest when the entire system is linked throughcentral linking groups involving multiple bonds (e.g., �CH=N� or�CH=CH�). But the central ester linkage does not link the systemthrough a multiple bond, and hence the mesogenic thermal stabilityof a system connected via a azomethine linkage is higher.

Reference to Table 4 indicates that the smectic mesophase rangeand smectic mesophase thermal stability of compound 3 is lower by9�C and 11�C respectively than those of compound B. The molecularstructural difference between compound 3 and B lies in the terminalposition. The ester tail is changed from a methoxy group for compound3 to a chloro group for compound B. The more polar chloro groupenhances the polarizability of the molecule of compound B and hencethe mesophase thermal stability. This is also reflected in the compari-son of compounds 15 and E where the smectic mesophase thermal

SCHEME 2


stability of compound 15 is lower by 40�C and the nematic mesophaserange as well as nematic mesophase thermal stabilities are lower by9�C and 49�C, respectively, than those of compound E.

From Table 4 we can see that the smectic mesophase range andsmectic mesophase thermal stability of compound 3 is lower by 2.5�Cand 7.5�C respectively than those of compound C. Both the compoundsdiffer at one terminus only. The ester tail is methoxyethyl for com-pound 3, whereas it is an n-butyl chain for compound C. Weygandet al. [13] have reported mesogenic properties of an alkyl chaincombining two ether functions as a terminal substituent, e.g.,CH3OCH2O�. Few compounds have been examined, but the datashow that the mesomorphic property disappears entirely or has lowernematic thermal stabilities than the analogous compounds containingthe group CH3CH2CH2O�. Attempts have been made to explain thiseffect in terms of differences in the linear nature of the two groups(one compound contains one and the other contains two oxygenatoms), but it is quite likely that the reason lies in some interactionbetween the two ether dipoles. For example, the effective dipolemoment of the group CH3OCH2O� may be a weaker resultant dipolealong the axis of the chain (Scheme 3).

Probably because of the same reason, the smectic mesophase rangeand smectic mesophase thermal stability of compound 3 is lower, as itcontains a broken alkyl chain (methoxyethyl chain) at the terminus,than for compound C, which has an n-butyl tail. Previously, we[16–20] have also observed that a broken alkyl chain at the terminusadversely affects the smectic thermal stability.

Table 4 shows that the nematic mesophase length of compound 15 islower by 45.5�C, whereas the nematic thermal stability is higher by12�C, than those of compound D. Table 4 also shows that the smecticmesophase length and smectic mesophase thermal stability ofcompound 15 is lower by 55�C and 23.5�C than those of compoundD. Reference to the molecular structure of both these compounds,indicates that both the compounds differ only at one terminal chain.In compound D one more methylene unit is present at the terminal

SCHEME 3 Partial dipole cancellation in the group �OCH2OCH3.


broken alkyl ester chain. The addition of the methylene groupdecreases the strength of the terminal intermolecular cohesions. How-ever, the addition of a methylene group does increase the polarizabil-ity of the molecule of compound D. Such increase in the molecularpolarizability tends to increase the smectic mesophase thermal stab-ility of compound D more than compound 15. We would also expectthe lateral intermolecular attractions to increase as the chain lengthgrows, thus compound 15 has a higher nematic thermal stability thancompound D, whereas addition of one methylene group adverselyaffects the smectic mesophase thermal stability, which agrees wellwith our earlier work [16,17].

4. CONCLUSION

In this article we have presented the synthesis and characterization oftwo new mesogenic homologous series of Schiff ’s base esters contain-ing methoxyethyl tails. Series I is purely smectogenic as it is a shorttwo-phenyl-ring system, whereas the compounds of series II exhibitthe nematic phase as well as the smectic A phase with good mesophaserange and higher thermal stabilities due to the presence of anadditional phenyl ring along with an ester linkage. The study indi-cates that though the broken alkyl tail is believed to be detrimentalto mesomorphic behavior, the compounds exhibit mesomorphic proper-ties with good thermal stabilities if properly designed.

ACKNOWLEDGMENTS

The authors are thankful to the dean, Faculty of Technology andEngineering, and head, Applied Chemistry Department, for providingthe research facilities.

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Mesogenic Schiff's Base Esters with a Methoxyethyl Tail

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